Joining of component parts together with a fused glass seal to form a composite article is a well cultivated art. In particular, numerous special sealing glasses have been developed for use in joining glass parts with each other, or with metals, alloys, or ceramics.
In making a fusion-type seal, the material must be heated to a temperature where it becomes soft enough to wet the sealing surface and form an adherent, hermetic bond. For many purposes, it is desirable to maintain the sealing temperature as low as possible. This is particularly true in electrical and electronic articles where thermally sensitive parts or coatings are commonly employed.
Accordingly, considerable attention has been given to lead glasses as low temperature sealing glasses. For example, stable sealing glasses, having softening points in the 430.degree.-500.degree. C. range and linear coefficients of thermal expansion over the 0.degree.-300.degree. C. temperature interval in the 70-90.times.10.sup.-7 /.degree.C. range, are disclosed in U.S. Pat. No. 2,642,633 (Dalton).
For many sealing and coating purposes, glasses are used in powder form, termed glass frits. One such application is forming a seal between the funnel and panel members of a cathode ray tube. Sealing glass frits are commonly mixed with an organic vehicle, such as amyl acetate, to form a flowable or extrudable paste. This mixture may then be applied to a sealing surface, e.g., the peripheral rim of a funnel or panel member. It is also known to incorporate mill additions in a glass frit mixture, a primary reason being modification and/or control of the ultimate coefficient of thermal expansion in a seal.
Industrial frit-sealing processes involve a secondary (post-seal) thermal treatment for metallizing, vacuum exhausting, or annealing. This second thermal step may be at a lower temperature than that of the sealing step. However, the temperature involved may be high enough to produce viscous deformation of the frit seal. This brings about distortion and misalignment of the sealed assembly.
As a solution to the problem, lead-zinc borate glasses were developed that, in frit form, thermally crystallized in situ. These frits remain relatively glassy for a period of time in the early part of a sealing step. This permits the glass to soften and flow to wet the sealing surface prior to crystallizing. Thereafter, with the sealing surface completely wet by the glass, the glassy frit crystallizes throughout to a rigid, deformation-resistant seal. This seal is capable of withstanding subsequent thermal processing, such as a vacuum bakeout, without distortion.
These thermally crystallizable lead-zinc borate, sealing glass frits have found broad application in the electronics industry. They are particularly useful in sealing together the panel and funnel members of a cathode ray tube. There is, however, a continuing search for improvements, both in the seal produced and in the sealing process.
The present invention provides specific mill additions to thermally crystallizable, lead-zinc borate sealing glass frits. These mill additions improve the mechanical strength of the seal produced. They also permit a faster sealing cycle by increasing the rate at which the glass frit crystallizes to form a rigid seal.